1. Field of the Invention
The present invention relates to a dielectric waveguide suitable for use in a transmission line or an integrated circuit which operates in a millimeter wave band or a microwave band.
2. Description of the Related Art
FIGS. 26(A) to 26(D) show, in sectional views, four types of conventional dielectric waveguides which are known as NRD waveguides (non-radiative dielectric waveguides). The waveguide shown in FIG. 26(A) is of the type which is generally referred to as the "normal type", and has a dielectric strip 100 and a pair of parallel metallic plates 101 and 102 between which the dielectric strip 100 is disposed. The waveguide shown in FIG. 26(B) is of the so-called "grooved type", and has a pair of grooved metallic flat plates 101 and 102 and a dielectric strip 100 received in the grooves of the flat metal plates 101, 102. The waveguide shown in FIG. 26(C) is of the type known as the "insulated type" in which a dielectric strip 100 is interposed between conductive plates 105 and 106 through intermediaries of dielectric layers 103 and 104 of a small dielectric constant. The waveguide shown in FIG. 26(D) is of the type which is referred to as the "winged type", and has a pair of dielectric strips 107 and 108 each having wings, and conductors 109 and 110 which are formed on flat portions of the dielectric strips 107 and 108, the dielectric strips 107, 108 being adjoined such that they face in opposite directions.
A dielectric waveguide of the normal type is disclosed in, for example, JP-B-62-35281. A dielectric waveguide of the grooved type is disclosed in JU-A-59-183002. A dielectric waveguide of the insulated type is disclosed in JP-B-1-51202. A dielectric waveguide of the winged type is disclosed in JP-A-6-260814.
These known types of dielectric waveguides have their own respective advantages offered by their structural features. These dielectric waveguides can operate in two transmission modes, one of which is the LSM mode while the other is the LSE mode. Usually, the LSM mode, in particular the LSM.sub.01 mode, is preferentially used because of its small transmission loss. A magnetic field distribution pattern peculiar to the LSM.sub.01 mode and a magnetic field distribution pattern peculiar to the LSE.sub.01 mode are shown by way of example in FIGS. 7(A) and 7(B), respectively. It is to be understood that conductors such as metallic flat plates disposed on the upper and lower sides of a dielectric strip 100 are omitted. Solid curvilinear lines with arrows indicate electric lines of force, while broken curvilinear lines with arrows indicate magnetic lines of force. FIGS. 8(A), 8(B) and FIGS. 9(A), 9(B) respectively show, by way of example, dispersion curves obtained with known dielectric waveguides of the normal type and known dielectric waveguides of the grooved type (FIGS. 8(A) and 9(A)) respectively, as well as calculation modes (FIGS. 8(B) and 9(B)). From these Figures, it will be seen that the LSE.sub.01 mode is the mode of the lowest order, and that the LSM.sub.01 mode, which is the transmission mode to be used, is of a higher order. This poses a risk that the LSE.sub.01 mode may unexpectedly occur regardless of the frequency when the LSM.sub.01 mode is being used. It is therefore necessary to take suitable measures for eliminating any influence which may be caused by occurrence of the LSE.sub.01 mode.
For instance, occurrence of the LSE.sub.01 mode takes place when the electromagnetic wave impinges upon a discontinuous portion of a dielectric strip 100 which exhibits lateral asymmetry of the LSM.sub.01 mode, as in the case of a bend as shown in FIG. 27. Although an upper metallic flat plate 101 is spaced from the dielectric strip 100 in FIG. 27, it will be clear that the plate 101 is assembled together with the dielectric strip 100 and a lower metallic flat plate 102 when the dielectric waveguide is subjected to use. The cut-off frequency in the LSE.sub.01 mode is lower than that in the LSM.sub.01 mode, so that the wave in the LSE.sub.01 mode propagates through the dielectric strip, causing a periodic repetition of a process in which part of the transmitted electric power of the LSM.sub.01 mode is converted into the LSE.sub.01 mode at the discontinuous portion and is then completely converted back into the LSM.sub.01 mode. It is therefore possible to minimize the loss at the bend, by designing the bend such that the electric power is fully converted into the LSM.sub.01 mode at the end of the bend. Conditions for achieving such a design, however, are extremely restricted and, therefore, it has been extremely difficult to construct a bend having a desired bend angle and radius of curvature.
FIGS. 28(A) and 28(B) show, by way of example, a circulator which is composed of three dielectric strips 100 and a pair of ferrite discs 32 and which operates under a D.C. biasing magnetic field H.sub.OC. When an electromagnetic wave of the LSM.sub.01 mode propagates from a port P1 to a port P3 as shown in FIG. 28(A), propagation of an electromagnetic wave of the LSE.sub.01 mode towards a port P3 also takes place, resulting in an increase of the loss. In FIGS. 28(A) and 28(B), broken-line loops show distributions of magnetic fields, and upper and lower conductors which also are components of the circulator are omitted. An effective measure for eliminating the undesirable influence of the LSE.sub.01 mode is to provide each dielectric strip with a mode suppressor 109a as shown in FIG. 28(B). The mode suppressor 109a is provided in its core portion with a conductor which extends vertically as shown, and is operative so as to suppress or attenuate only the LSE.sub.01 mode. This measure, however, is not recommended, since it requires provision of suppressors which occupy considerable space.
Another problem is that, when it is desired to arrange, for example, a pair of dielectric strips in a mutually crossing manner, these strips have to be disposed at different heights or levels in order to eliminate interference between the electromagnetic waves propagating through these strips. Such a three-dimensional arrangement undesirably increases the dimensions of the whole device.